The present invention relates primarily to the field of manufacturing of semiconductor devices and particularly to the field of producing unfired ceramic sheets, used in making ceramic substrates, which can be processed by automated equipment.
In the manufacture of ceramic sheets for use in making VLSI circuits, a slurry is first made which consists of ceramic particles such as alumina or glass particles which are suspended in an organic solution containing an organic binder including plasticizers. The slurry is then deposited in a thin layer on a backing web made, for example, of MYLAR flexible polyester film material (MYLAR is a registered trademark of E. I. DuPont De Nemours & Co. of Wilmington, Del.). The slurry on the web then passes under a forced air dryer to drive off the solvents in the slurry thereby leaving a thin layer (ca. 5 to 30 mils in thickness) of unfired ceramic material embedded in a solid organic matrix. The ceramic-containing layer is subsequently separated from the backing web and wound in the form of an unfired ceramic web onto a spool. Thereafter, the spooled unfired ceramic web is cut into small sheets which are preferably loaded into a cartridge which holds and separates a plurality of such sheets which are referred to as "green sheets", a generic term used to describe a ceramic sheet prior to firing or sintering.
It is also known tnat green sheets are unstable as has been noted in U.S. Pat. Nos. 4,497,677 and 4,340,436. Indeed, a frequently encountered instability is that of edge curl. Green sheets frequently curl up at their edges primarily as a result of the forced air dryer for removing solvents from the slurry used in making the green sheets. The extent of curling which is experienced in green sheets is related to the particular ceramic material, the organic matrix in which the ceramic material is embedded, the particular solvents which are used to make up the slurry, and the speed with which the solvents are removed from the slurry.
The curl in green sheets, if it is large enough, can cause problems with certain types of automated equipment designed for handling such green sheets. For example, a machine designed to cut the rolled unfired ceramic web into sheets has been found to be sensitive to edge curl in that the sheet transport does not work well with sheets having an edge curl greater than about 20 mils. Similar problems can occur with vacuum pickup mechanisms if the green sheet is not flat. Problems may also develop in green sheet punching if the sheet is not flat.
Various solutions have been suggested to solve the problem of edge curl on green sheets. As the edge curl is primarily caused by the speed at which solvents are removed from the slurry used in forming the web from which the green sheets are cut, one solution is to slow the speed at which the solvents are removed. This approach, however, has the disadvantage of slowing the manufacturing process used in forming the web thereby unnecessarily tying up expensive manufacturing equipment.
An alternative approach is to "iron" out the curl. In this approach, a weight is placed on the green sheets with edge curl. Then the temperature is raised for a period of time. The elevated temperature is not sufficiently high to cure the ceramic material but is sufficiently high to relax the stresses in the sheet. This approach, however, has been found to contribute to a decrease in screening stability of the sheet. Accordingly, this approach is not desirable, especially in components having high circuit density.